The advent of DWDM fibre optics telecommunications systems in the early 1990s have enabled a dramatic increase in the transmission capacity over point-to-point links. This was achieved through multiplexing of a large number of individually modulated light beams of different wavelengths onto the same optical fibre. Typical systems installed today would have 64 or more independent channels precisely aligned onto an ITU-T standardized grid at 100 GHz, 50 GHz or even narrower channel spacing. With routine modulation speeds of 10 Gb/s and attaining 40 Gb/s in field trials, it is not unusual to obtain aggregated capacities in the order of several terabits per second of information being transmitted onto a single optical fibre (S. Bigo, Optical Fibre Communications conference, WX 3, pp. 362–364, Anaheim, 2002). At the same time, electrical switching capacities have been growing at a much slower rate, with the largest current electrical switch matrices limited to typically 640 Gb/s in single stage. Furthermore, the cost of converting the signal from optical to electrical for switching and then back from electrical to optical becomes prohibitively expensive as the number of optical channels increases. All optical switching technologies are therefore becoming more and more attractive to manage this enormous bandwidth.
An all-optical switch would consist of a large core optical switching matrix surrounded by DWDM demultiplexers and multiplexers at the fibre interface. However, for a large number of wavelength channels per optical fibre, this leads to a very large switching core size: for example, a 50 GHz channel spacing system with 128 channels per fibre would require a 1024×1024 switching matrix to switch traffic between 8 incoming fibres and 8 outgoing fibres on a per wavelength basis. Large optical switching matrices are hard to fabricate, complex to control, require overwhelming fibre management and are very expensive. Furthermore, in the absence of wavelength conversion, only a sub-set of the switching matrix capacity is actually in use: with each wavelength being switched independently, only 128 8×8 independent connections are used in the 1024×1024 available (0.8% of the overall switching capacity). This huge inefficiency is the primary reason for considering a wavelength switching architecture in which the DWDM demultiplexing and multiplexing are integrated with the switching function.
Both free-space optics (J. E. Ford et al., Journal of Lightwave Technologies, Vol. 17, No. 5, May 1999) and waveguide optics (M. Katayama et al., Optical Fibre Communication conference, WX4, Anaheim, 2001) embodiments have been proposed in the past. So far, free-space optics embodiments have enabled the highest optical performance in terms of spectral efficiency, with for example, 85 GHz full width at half maximum passband for 100 GHz spacing (D. T. Neilson et al., Optical Fibre Communication conference, ThCC3, pp. 586–588, Anaheim, 2002). However, to obtain this level of spectral efficiency requires an array of actuators (in the case of this last reference, MEMS micro-mirrors) with a very high fill factor. This poses severe constraints on manufacturing and on long term reliability, due to increased risk of lateral stiction from neighbouring mirrors for example in the case of MEMS. Furthermore, the finite gap between actuators shows as “dips” in the spectrum, even when consecutive switching elements are not actuated. Upon multiple cascades in the network, these dips could cause signal degradation.
It would therefore be advantageous to have a wavelength selective switch in which a low fill factor actuation array can be used while maintaining high spectral efficiency.